XenForo Need help with the fallowing code

[eL_]

Code:

body:not(.logged-in) .message--post {
   filter: blur(10px);
   -webkit-filter: blur(10px);
   position: relative;
}

body:not(.logged-in) .message--post:before {
   content: "";
   background-image: url("https://example.com/blurred-background-image.jpg");
   background-size: cover;
   background-position: center;
   background-repeat: no-repeat;
   width: 100%;
   height: 100%;
   top: 0;
   left: 0;
   position: absolute;
   z-index: 1;
}

I don't know what I'm doing wrong, what I...

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